The invention relates to a process for producing a dimensionally stable fibre-reinforced thermoplastic semifinished product.
The reinforcing fibres are in particular constituted by cut glass fibres, but it is also possible to use other mineral, inorganic or artificial fibres, such as carbon, aramide or polyester fibres. When reference is made hereinafter to glass fibres, they can be replaced by the other aforementioned fibres. Cut glass fibres are used in large quantities for producing fibre glass-reinforced plastic parts. The glass fibres are bonded by a binder, which as the main constituent or matrix has a thermoplastic material, such as polypropylene, as well as additionally carbon black, wax and other additives and chemical couplers. A semifinished product compressible to a moulding in a press takes place by strewing the fibres to form a fleece and impregnating the same with liquid synthetic resin. In another form, the fibres are processed in a liquid phase to a suspension, which is processed to a tangled fibrous material by stirring or mixing movements. Here again liquid synthetic resins or pulverulent binders are used in conjunction with an aqueous suspension. In the latter case, the suspension is dried after forming a tangled fibre fleece. In both cases the thickness and width of the fleece must be adapted to the mouldings to be produced, the former being necessary and the latter ensures that there is not excessive waste. The fleece must then be cut into individual plates to adapt to the mouldings to be produced.
It has also already been proposed (German application P 36 04 888.7) to add liquid wetting agents with a proportion of max 20% by weight, in order in this way to obtain moist, but still free-flowing material, which is packed as an intermediate product in plastic bags and forwarded to the further processing enterprise, or can optionally be processed in a heated belt press to preshaped plate material.
In particular, the first-mentioned processes are very complicated and involve high costs. In all the aforementioned processes preparation and compounding of the semifinished product are required, which makes the processes inflexible and prevents or makes very difficult adaptations to other mouldings.
It has also already been proposed to whirl glass fibre bundles together with the thermoplastic material-containing binders in a turbulence or whirling chamber to form a cotton wool or wadding-like belt and then to pack same under vacuum, preferably in plastic bags made from material compatible with the thermoplastic material of the binder and in particular identical therewith, or to make same directly available for further processing (German application P 37 04 035.9).
On the basis thereof, the aim underlying the present invention resides in providing an easy handling, effortlessly transportable and largely automatically further processable semifinished product without high energy costs being required, particularly in such a way that there is no need for preparation and compounding of the starting material as a function of the mouldings to be produced.
According to the inventive process, a pourable initial product constituted by fibres and thermoplastic material-containing binders is heated to above the melting point of the thermoplastic material only in a thin covering area. An inventive apparatus, particularly for performing the process, is characterized in that there is a melting device with a melting channel, through which the material to be processed is pressed as a continuous strand and a cooling device is connected to the melting device. A semifinished product fulfilling the indicated requirements is characterized by a solid covering of fibres and thermoplastic material resolidified after melting and a felt or cotton wool-like core of a fibre-polymer mixture.
The inventive process makes it possible to simplify the production sequence, as well as ensuring a high degree of automation during the production of mouldings. For further processing purposes, there is no need for a preparation and compounding of the semifinished product adapted to the end product. Therefore costs can be kept low. Through constructing the inventive apparatus as a closed system it is possible to prevent decomposition of the thermoplastic matrix by overheating, so that there is a reduced fire risk and better environmental conditions. The inventive process permits considerable variation possibilities with regards to the use of higher grade thermoplastics (e.g. PA, PTP) and the use of different reinforcing fibres. In particular, the inventively produced semifinished product can be preheated for further processing by inert hot gases or hot air blown through the felt or cotton wool-like core to below the melting point of the polymer or can be heated to above its melting point, which is not very energy-intensive, particularly when recirculating the heating gas.
The starting material can in particular be supplied in a standard pack size and need not, as is the case for glass mat production, be adapted according to the special charge weights, etc. of the mouldings.
If not adequately compressed material, as is the case when using the material produced according to the prior art, but instead the aforementioned cotton wool-like felt is directly supplied, according to a preferred embodiment of the invention, the cotton wool-like initial product is precompressed and directly supplied to the continuous strand or directly upstream of the melting device is provided a compressing station. According to a further development the compression station has a cooled compressing channel, to which is directly connected the melting channel of the melting device and that at the end of the compressing channel remote from the transition point from compressing channel to melting channel is movably arranged a compressing and feed plunger and in particular that the melting channel of the melting device is surrounded by heating elements over its entire length.
According to a preferred construction a vacuum is applied when compressing the starting material, so as to avoid air inclusions and the like. In particular in the vicinity of the compressing station, it is possible to provide vacuum lines by means of which the vacuum is applied. According to a preferred development the melting channel is provided over its entire length with heating elements. The heating of the material in the melting channel takes place to a temperature, which is higher than the melting point of the thermoplastic material and is in particular up to 100.degree. C. above the melting point. The inventive process permits a very high throughput of material to be melted, so that it is readily possible to achieve a throughput of 2 kg or more per minute.
The wall of the melting channel is briefly heated by its heating elements to well above the melting point of the polymer of the starting product, so that the polymer only melts in a relatively thin covering area, but not in the inner core of the wadding-like fibre-polymer material located in the channel. This is followed by cooling, so that the melted polymer resolidifies and is plasticized. It is important to have a rapid heating, so that the outer area of the polymer melts, without the high temperature passing into the core area and there is advantageously a subsequent, fast cooling.